Within the next decade, x-ray imaging systems will be replaced by imaging systems using terahertz frequency sources and detectors in areas such as medical, security and quality control applications. Terahertz frequency waves, or t-rays, can penetrate most solid substance like x-rays. In contrast to x-rays, t-rays are non-ionizing, and thus are non-lethal and safer for imaging applications. Further, t-ray systems produce true high resolution images rather than shadowy images produced by x-ray systems.
A heavy demand for terahertz technology also exists in the communications industry. Development of components necessary for a terahertz frequency heterodyne receiver will result in a dramatic increase in the available bandwidth in wavelength-division-multiplexed communications networks.
The terahertz band of the electromagnetic (E-M) spectrum exists between the mid-infrared band and the microwave band. Loosely defined, the terahertz band encompasses that part of the frequency spectrum that includes the frequencies ranging from about 0.3-10.0 terahertz (THz), or equivalently, the wavelengths ranging from about 1.0-0.03 millimeters. In the art, the terahertz band is also known as the far-infrared band or the submillimeter band.
The terahertz band is one of the last spectral regions to have compact, powerful, coherent sources available. Compact, high-performance t-ray systems, such as imaging and communication systems, need powerful, solid-state, pulsed and/or continuous-wave t-ray sources. Also, tunable, narrow-band, continuous-wave t-ray sources are necessary to produce high-performance terahertz frequency heterodyne receivers.
Until recently, only thermal incoherent optical sources emitted a significant amount of light in the far-infrared (FIR) or terahertz band of the frequency spectrum. Within the last few years, several types of FIR coherent optical sources have been developed for pulsed and continuous-wave applications. These FIR coherent optical sources include direct coherent sources (DCS), electronically-mixed electronic oscillators (EMEO), electronically-mixed optical oscillators (EMOO), and optically-mixed optical oscillators (OMOO). Each of these prior art optical sources methods has disadvantages.
DCS must support a resonant interaction between two energy levels spaced extremely close together because the FIR band is an extremely low energy band in comparison to the near-infrared and visible bands of the electromagnetic spectrum. Direct laser sources supporting the resonant interaction described are either vapor lasers or cryogenically cooled solid-state devices. Two types of DCS include the methanol laser and the heavy-hole light-hole laser.
The methanol laser is a gas laser pumped by another gas (CO.sub.2) laser, and provides a moderate power level. However, when compared to a solid-state device that may have a similar output power, the methanol laser is inefficient, bulky, unreliable and difficult to maintain. Despite the drawbacks, the methanol laser remains in operation because a continuous-wave, solid-state device having a similar output power does not yet exist.
The heavy-hole light-hole laser is a solid-state laser in which the dominant transition between energy levels occurs between the heavy-hole band and light-hole band of a semiconductor. Since the transition between energy levels amounts to such a low energy, the laser crystal must be cryogenically cooled to prevent thermal electrons from adding spurious light to the output signal. Also, the output power of the heavy-hole light-hole laser is very low and will likely not increase substantially through further development.
Another method of producing a coherent light source uses electronically mixed electronic oscillators (EMEO). Employing a Gunn oscillator/multiplier, EMEO multiplies the frequency of a stable electronic oscillator using electronic mixers. Typically, the input frequency is multiplied by a factor of two or three in a single stage. To obtain the desired terahertz frequency from the fundamental oscillator frequency, many stages are required. However, such stages are inefficient and EMEO, to date, has been limited to frequencies less than 300 GHz.
Electronically mixed optical oscillators (EMOO) are a type of coherent terahertz source that is produced by mixing laser oscillators in photoconductors. Since the photon energy of a laser source must be greater than the band gap energy of the photoconductor, the laser excites electrons into the conduction band when the laser is incident on the photoconductor. The electrons in the conduction band (i.e., free carriers) cannot respond directly to the fundamental frequency of the incident laser, which is on the order of hundreds of terahertz. However, the free carriers can respond to frequencies on the order of tens of terahertz or less. Consequently, two distinct possibilities emerge, one for a narrow-band terahertz source and another for a broadband terahertz source.
A narrow-band terahertz source includes two narrow-band lasers. Each laser has a photon energy above the band gap energy of the photoconductor and a beat frequency, or frequency difference, as the terahertz frequency generated. The incident photons produce photoelectrons that respond to the beat frequency of the two incident lasers.
A broadband terahertz source includes one source laser having its photon energy above the band gap energy of the photoconductor to produce photoelectrons upon incidence. Rather than having two narrow-band lasers provide the necessary terahertz beat frequency, the one source laser has the necessary terahertz bandwidth to directly produce the terahertz frequencies to which the photoelectrons can and will respond. To achieve this operating capability, the broadband source employs lasers with sub-picosecond pulse lengths.
Presently, the broadband source, by mixing short-pulse (e.g., sub-picosecond pulse lengths) lasers in photoconductors, is far more developed than any narrow-band, continuous-wave source. This condition exists because the terahertz output power strongly depends on the peak power of the incident laser, and currently, the peak power of sub-picosecond lasers is much greater than the peak power of continuous-wave lasers. The most commonly used photoconductors for EMOO are III-V semiconductors, and in particular GaAs.
The broadband source method involves electronic rectification of sub-picosecond laser pulses. An extremely short laser pulse is incident on a photoconductor, and by design, the band gap energy of the photoconductor is less than the average photon energy in the incident laser pulse. The photon energy in the incident laser pulse excites electrons from the valence band of the photoconductor into its conduction band. Free carriers are generated instantaneously and the lifetime of the generated electron-hole pairs is much longer than the optical (i.e., laser) pulse length. Therefore, once generated, the free carriers can respond to the remaining electric field due to the incident optical pulse. The electric field accelerates the particles causing the particles to reradiate and rectify the terahertz bandwidth pulse. This creates a moderate amount of output power in the terahertz region. The output beam contains all the frequency components of the rectified optical pulse, and not merely the terahertz components. Unique to EMOO, a free space terahertz beam is produced, whereas other methods require coupling to an emitting antenna to obtain a free space beam.
A drawback of the broadband source method is that it is inherently self-limiting. The interaction in the semiconductor is limited to a length on the order of the far-infrared absorption length. The far-infrared absorption length limits the total mode volume, which directly affects the obtainable output power. However, doping increases the far-infrared absorption, thus reducing the far infrared absorption length, the interaction length, the mode volume, and ultimately the output power. This becomes a problem because the semiconductor must be doped with impurities to generate a sufficient number of free carriers when the optical pulse is incident.
Two narrow-band source methods exist. The methods include difference frequency mixing of continuous-wave diode lasers in 1) low-temperature-grown (LTG) GaAs photoconductors according to one method or 2) metal-semiconductor-metal, micro-structured GaAs photoconductors according to the other method. In both methods, the photon energies of the two lasers being mixed must be above the GaAs band gap.
The LTG GaAs photoconductor contains many impurity-associated electronic traps. When the optical fields are incident, the trap ensures that a photo-generated electron will be within a small distance of the photoconductor electrode. Consequently, there is a short transit-time from the point where the free carrier is created to the collection electrode, thereby allowing the device to respond to terahertz frequencies. However, this method is self-limiting because it attempts to drive a large current produced from the photo-generated electrons through a highly resistive nonlinear device, i.e., an LTG GaAs photoconductor with electronic traps. The output power is less than 0.5 microwatts when the device experiences thermal burnout at a diode laser pump power of about 100 milliwatts. Also, a coupling beam is required for a free space beam.
The method using metal-semiconductor-metal, micro-structured GaAs photoconductors involves mixing two continuous wave lasers within metal-semiconductor-metal, microstructured photoconductors (MSMMP) using impurity-free GaAs. In contrast to the method using LTG GaAs, a small gap between the two electrodes where the photo-carriers are generated ensures a small transit time for the photo-carriers rather than the creation of traps in the photoconductor material to act as centers of photo-carrier generation. The MSMMP method is similar to the optical rectification method described above with two primary distinctions. In the MSMMP method, the incident lasers are narrow-band, continuous-wave diode lasers, which have a much lower intensity than the picosecond lasers employed in the rectification method. Also, the MSMMP method does not produce a free space beam and thus requires coupling to produce a free space terahertz beam. The MSMMP method has yet to be demonstrated and may suffer the same thermal burnout problem of the LTG GaAs method because both methods generate a large photocurrent over a short distance. However, the MSMMP device does not possess the traps associated with the LTG GaAs device, and may be less susceptible to thermal burnout. If so, the output power may continue to scale until incident lasers reach the optical damage threshold.
Another series of methods for coherent light source production employ optically mixed optical oscillators (OMOO). This field has been given the name of nonlinear optical frequency conversion and applies to any frequency conversion, up or down. The natural oscillation frequency of most solid-state lasers is much larger than 10.0 THz. Thus, for terahertz applications, down-conversion of solid-state laser oscillation frequencies is of primary interest. This nonlinear effect is referred to in the art as difference frequency mixing.
One OMOO method involves the rectification of sub-picosecond laser pulses in nonlinear optical crystals, which is similar to the EMOO rectification method described above. Although this method employs the same type of short pulse laser as a pump, and the nonlinear element is a semiconductor crystal, this rectification method differs in that the average photon energy is less than the band gap energy of the photoconductor. Thus, the incident laser energy does not excite electrons from the valence band of the photoconductor into its conduction band; rather the method utilizes the nonlinear optical properties of the crystal. As a nonlinear optical process, an efficient interaction requires velocity matching between the phase of the pulse and the phase of the generated terahertz wave. Velocity matching causes the coherence length of the interaction to be long. Group velocity matching of a pulse to a terahertz wave has been achieved in ZnTe using a single Ti:sapphire picosecond pulsed laser operating at approximately 0.800 micrometers as a pump source. As with the electronically mixed rectified short laser pulse, the output beam contains all the frequency components of the rectified optical pulse, and not merely the terahertz components. Also, this method produces a free space terahertz beam.
One of the disadvantages of this rectification method is that it uses only one laser for a pump source. For a wide-band terahertz source, the rectification method employing a single source cannot maintain a large coherence length over as wide a bandwidth as a three-wave mixed system with two slightly-frequency-separated input sources. Theoretically, the pulse bandwidth could be substantially widened, that is the pulse could be made even shorter. However, many problems would accompany such a modification. For example, constructing a shorter pulse pump laser would be extremely difficult and expensive. Also, as the pump pulse becomes shorter in time, the physical length of the pulse becomes smaller and synchronizing the entire optical system becomes extremely difficult. This leads to very small tolerances on optical alignment and timing. Further, since this method produces all frequency components, it is less efficient than a two input source system that produces only terahertz waves. Also, the rectification method cannot produce a narrow-band terahertz source.
Another type of OMOO involves three-wave mixing in nonlinear optical crystals. In a three-wave nonlinear interaction, the overwhelming consideration driving the efficiency of the interaction is the need for phasematching. Types of prior art phasematching techniques include birefringent phasematching, noncollinear phasematching, and quasi-phasematching.
Attempts to generate terahertz waves using birefringent phasematching in birefringent crystals were initiated in the 1970s using CO.sub.2 and ruby lasers and various birefringent crystals, and abandoned until recently. Recently, terahertz waves were generated using birefringent crystals with a pulsed Nd:YAG laser. As a frequency converter element for generating terahertz waves, birefringent crystals have two main disadvantages.
Most important, birefringent crystals tend to absorb strongly in the far-infrared band of the spectrum due primarily to their many fundamental modes of vibration, which leads to a very wide Reststrahlen band. Although the cause of the far-infrared absorption is different, the end result is the same as the free electron absorption experienced by the photoconductors. The crystal absorbs much of the terahertz radiation that it converts.
Also, all birefringent crystals suffer from "walk-off". "Walk-off" refers to the phenomenon of double refraction in linear optics, and in both linear and nonlinear optics, birefringence of the crystal causes double refraction. In laymen's terms, the interacting pump and signal "beams" begin to separate in space causing the region of overlap of the two beams to shrink. Since the beams begin to separate, the conversion efficiency of the interaction shrinks as the beams travel through the length of the crystal. For purposes of analysis, the walk-off angle is defined and calculated which leads to the calculation of an effective interaction length. The effective interaction length is finite and can be quite small, depending on the birefringence of the crystal and the orientation of the crystal when oriented at the desired phasematching angle (i.e., depending upon how much of the birefringence of the crystal is needed to obtain the phasematching condition for the interaction).
The method of noncollinear phasematching obtains the phasematching condition by having the incident pump and signal wave sources with slightly different propagation directions in the nonlinear crystal. As a result, altering the angle of the projection of the wavevector along the phasematching direction can vary the length of the wavevector for a certain wavelength in a specific direction. Typically, noncollinear phasematching is reserved for nonlinear interactions in which a birefringent crystal is unavailable. Also, noncollinear phasematching can be used to increase the acceptance angle of the nonlinear crystal by judicious choice of the noncollinear angle. A disadvantage of noncollinear phasematching stems from the condition that the beams are noncollinear. Consequently, the beams will separate as they travel through the crystal and thus, the interaction length will be finite.
The method of quasi-phasematching obtains the phasematching condition by creating a periodic structure with the nonlinear material to compensate for any resultant phase mismatch between the interacting waves. In ferroelectric nonlinear crystals, the periodic structure can be created with a large electromagnetic pulse and a series of periodic electrodes on opposite crystal faces. In all other nonlinear crystals, the periodic structure can be created by literally "cutting and pasting". Specifically, the nonlinear crystal is cut into equal length pieces and both sides of each are optically polished. Next, every other piece is "flipped" (reoriented) 180 degrees around the axis of propagation. After polishing and reorientation, all the pieces are diffusion bonded back together with such precision that the new interfaces appear transparent to the interacting waves. A disadvantage of quasi-phasematching is the exorbitant manufacturing cost of creating the periodic structure.
In any nonlinear optical mixing technique, once the frequencies of the interaction have been chosen, the efficiency of transferring power from the input (infrared) frequency to the output (terahertz) frequency scales with three major contributing factors. The factors are 1) the intrinsic optical properties of the nonlinear crystal performing the frequency conversion process; 2) the optical intensities of the incident (infrared) light source (along with the optical damage threshold of the crystal); and 3) the length, within the nonlinear crystal, over which frequency conversion takes place. When compared to the EMEO and EMOO methods described, the OMOO method has the greatest potential for producing a powerful terahertz source because the performance of a bulk nonlinear crystal with respect to the first two factors far exceeds the performance of the nonlinear elements used in the EMEO and EMOO methods. However, the difficulty of maintaining an efficient conversion process over a sufficiently long length of crystal makes the OMOO method unattractive when compared to the EMEO and EMOO methods. In the OMOO method, since the nonlinear crystal does not absorb terahertz radiation, the length of the crystal is significantly impacted by the required phasematching condition that must be satisfied to achieve an efficient frequency conversion process.
Each of the above-described methods of providing a coherent optical terahertz source has drawbacks. Thus, there is a need for a compact, powerful, coherent terahertz source that does not suffer from the above drawbacks, and more particularly, a need for a phasematching technique that does not suffer from the disadvantages of the prior art phasematching techniques.